Such attacks are required in particular in the manufacture of semi-conductor components which may be constituted, for example, of silicon or of silicon carbide SiC and on which a protective layer has been initially deposited. The layer may be of silica or of silicon nitride for example, and windows need to be cut therethrough.
It may also be necessary to attack a thin film of semi-conductor material itself.
Numerous methods of attack are known: aqueous phase chemical attack; ionic attack in a chemically inert gas without chemical decomposition; and gas phase chemical attack at high temperature, above 1000.degree. C. The known ionic attack method is called "cathod sputtering." The material which is to be attacked is introduced in the form of a solid target. A direct electric field is established between the target and a plasma which is created on site from a gas by means of a high frequency field. The field causes the target to be bombarded with ions, thereby tearing atoms away from the target by means of cathode sputtering. This method may also be used to deposit target material onto a substrate placed in the neighborhood of the target.
The quality of the deposit can then be improved by electrically polarizing the substrate with respect to the plasma, for example by means of a connection between the substrate and the target, provided the target constitutes one of the conductive surfaces between which the high frequency field is applied, thereby enabling the direct electric field to be established by the high frequency source. In this case, the substrate is connected via an impedance, for example a variable capacitor as described in an article by Orla Christensen et al. "RF biasing through capacitive collector to target coupling in RF diode sputtering" Journal of Physics E.: Scientific Instruments 1972 vol. 5, printed in Great Britain.
Finally low temperature gas phase methods of chemical attack are known in which a gas such as carbon tetrafluoride CF.sub.4 is decomposed in a plasma created by a radio frequency field. The decomposition results from the molecules being broken by the electron collisions produced in the plasma. One of the decomposition products, e.g., fluorine ions, chemically attacks the film to be attacked, e.g., silica. This method of chemical attack by decomposition of a gas in a plasma has the drawback of low geometrical definition of the attacked zones:
The zones which should not be attacked are protected by a layer of resin through which there are windows defining the zones which are to be attacked. Now the zone which is attacked extends beyond the edge of the window and penetrates beneath the layer of resin to a distance which cannot be accurately forecast. This phenomenon is called under-etching.
Another drawback of this known method is that the rate of attack is slow. The rate can be increased a little by increasing the gas pressure, but this results in an increase in the underetching. It may also be increased by increasing the radio frequency power, but that can cause the protective layer of resin to come unstuck, on account of its becoming heated by electron bombardment. In practice, acceptable definition in the order of 0.1 microns can only be retained by accepting a rate of attack which is limited to about 20 to 30 angstroms per minute.
Hewlett-Packard's French Pat. No. 2,253,280 which corresponds partially to U.S. Pat. No. 3,971,684 describes a method of attack by decomposition of a gas in a plasma. It aims to obtain both a high rate of attack and precise definition in the attacked zones. To do this it biases the substrate negatively with respect to the plasma. This bias stems from the fact that one of the terminals of the HF generator is connected to the substrate. Nonetheless this method does not completely eliminate the above mentioned drawback relating to the rate of attack. Also it is difficult in their method to chose an appropriate negative potential for the substrate since it stems from the application of the HF voltage which has to be chosen to be appropriate for the formation of the plasma.
The aim of the present invention is to provide a method of attacking a thin film by decomposition of a gas in a plasma, which enables both a high rate of attack and a precise geometric delimitation of the attacked zones to be obtained.